For implementing radio multimedia, it is necessary to multiplex and transmit media information such as image data, voice data, additional data, and the like. Especially for giving and taking these kinds of information by using a mobile communication terminal, it is important that the information is made to be capable of being transmitted in a poor environment such as a multi-path fading environment.
Heretofore, ITU-T recommendation H.223 has been standardized as a scheme concerning multimedia multiplexing. This scheme implements multimedia multiplexing of packet multiplexing type in an existing telephone network. An example of H.223 is shown in FIG. 13A. In FIG. 13A, LCN represents logical channel, AL adaptation layer, PM packet marker, MUX and multiplexing.
Typically, in a MUX packet, a header is disposed at its head. In succession, four voice bytes (LCN1), one data byte (LCN2), two image (video) bytes (LCN3), one data byte (LCN2), and two image bytes (LCN3) are disposed in the cited order. In the example of FIG. 13A, however, image data finishes in the middle of a MUX packet, and consequently only one byte is accommodated in a two-byte capacity in the last LCN3. This is indicated by setting a PM bit in the next packet header to “1”.
The format of the header is shown in FIG. 13B. With reference to FIG. 13B, by referring to an entry of a multiplex table in a four-bit MC (multiplex code) field, it is specified which media information is represented by each byte of an information field. A three-bit HEC (header error control) field provides an error detection function of the MC field using a three-bit CRC. (As for details, see “ITU-T Draft recommendation H.223, for example.)
By the way, H.223 has been determined on condition that multimedia multiplexing of packet multiplexing type is implemented in an existing telephone network having a comparatively fine transmission quality as described above. In order to raise the transmission efficiency, the header is protected by the three-bit CRC alone.
In radio multimedia communication, however, the transmission channel state is made poor by fading or the like. If it is attempted to apply H.223 to radio multimedia communication as it is, therefore, then such a situation cannot be coped with by the CRC of three bits or so, and header errors frequently occur. This results in a problem that contents of the multiplex table cannot be read and discard of MUX packets frequently occurs.
Furthermore, the length of the MUX packets is not always constant, but changes according to the information content of each media information as shown in the example of FIG. 13A as well. If packets of such a variable length are transmitted through a poor radio transmission channel, then packet synchronizing cannot be attained or the packet length cannot be found on the receiving side, resulting in frequent discard of MUX packets.
On the other hand, payloads containing information such as data, voice, and data cannot be decoded correctly either regardless of the received result of header information, if the radio transmission channel is brought into a poor state. Heretofore, therefore, there has been proposed such a scheme as to protect the payload by applying convolution encoding to information of each of image, voice, and data. (As for details, see, for example, “Proposal for High Level Approach of H.324/Annex C Mode 1”, Q11-A-11b, ITU-T Q11/WP2/SG16, June 1997.)
If it is attempted to securely protect the information of the payload, however, it is necessary to encode all of the information to be protected, resulting in a lowered transmission efficiency. This is a serious problem especially in a system having a limited transmission band such as a mobile communication system.
As a scheme for putting information data of a plurality of kinds such as multimedia information into a packet and conducting multiplex transmission, there is a scheme standardized on condition that transmission is conducted via a wire telephone network as heretofore described. If this standardized scheme is employed as it is in a radio communication system, however, detection errors of header information are frequently caused on the receiving side by the poor transmission channel state. Since the multiplex table cannot be read, discard of packets frequently occur. Especially in the case where the packet length is variable, there occurs such a state that the packet synchronizing cannot be attained or the packet length cannot be recognized, resulting in a substantially disabled state in communication.
On the other hand, for the payload, there has been proposed such a scheme as to protect it by using an error correction code such as a convolutional code. If it is attempted to decode information securely on the receiving side by using a conventional scheme, however, the transmission efficiency of the information significantly falls. This poses an especially serious problem in a mobile communication system in which it is difficult to secure a wide transmission band.